Precision wire wound resistors



NOV. 1, 1966 J ZUK I 3,283,285

PRECISION WIRE WOIJND RESISTORS Filed March 27, 1963 2 Sheetsheet 1 INVENTOR. JOHN ZUK ATTORNEYS PRECISION WIRE WOUND RESISTORS v Filed March 27, 1963 2 Sheets-Sheet 2 INVENTOR. JOHN ZUK BY ATTORNEYS United States Patent 3,283,285 PRECISION WIRE WOUND RESISTORS John Zuk, 90-06 Springfield Blvd, Jamaica, N.Y. Filed Mar. 27, 1963, Ser. No. 268,310 20 Claims. (Cl. 338299) This invention relates to precision wire wound resistors and more particularly to resistance trimming arrangements for precision wire wound resistors.

As is known, there exists a class of resistors known as the Wire wound type. These resistors have the general characteristic of being able to be constructed in a manner so that a desired amount of resistance can be obtained with a very high degree of precision. Typical applications of use for such precision Wire wound resistors are in computer circuits, voltage dividers, test instruments, etc.

In the normal construction of a wire wound resistor, a certain type of wire is selected which has a specified resistivity and resistance per unit length. This wire is then wound around a suitable form such as a bobbin with the number of turns necessary to obtain the desired amount of resistance. The ends of the resistance wire are connected to respective leads, which are usually greater in diameter than the diameter of the resistance wire, and the leads are used to connect the Wire wound resistor in the circuit.

Since it is almost impossible to wind the exact number ofturns of wire to achieve the desired accuracy of resistance, which, for example, may be about one part per million, some arrangement must be provided to finally adjust or trim the resistor to its desired value. This trimming arrangement must be capable of obtaining the desired accuracy and it should preferably be simple in nature.

One method for the final adjustment or trimming of a wire wound resistor which is performed in two steps is described in detail in the National Bureau of Standards Circular #470, published in 1948. In this method, excess resistance wire is first cut off the bobbin to make the total value of resistance of the wire slightly less than the nominal desired value. Copper lead wires are then silversoldered to the ends of the resistance wire. These lead wires are generally of larger diameter than the resistance wire. Final trimming of the resistor to its desired value is obtained by grinding, filing or scraping the thinner resistance wire on the bobbin near its ends where it is connected to the lead wires. This last step increases the resistance of the wire and it is continued until the overall resistance of the wire is increased to the desired value.

The above described method of trimming a wire wound resistor has several disadvantages. First of all, it is extremely diflicult to scrape or file small diameter resistance Wire which is tightly wound on a bobbin. Additionally, filing or scraping the resistance wire can produce only a relatively narrow range of trimming resistance variation which might not be enough in a particular application such as in high ohmic value wire wound resistors.

In another type of resistance trimming arrangement, the resistor is actually wound in two parts. First, there is a main resistance winding which is wound over the main part of the bobbin. This resistance of this main winding is usually made slightly less than the desired overall value of resistance and a trimming resistance is added. The trimming resistance is made of the same or diflerent diameter wire as the main resistance winding. Suflicient trimming wire is added in series with the main resistance wire to bring the overall resistance up to the desired value. In some cases the trimming wire is wound on a separate bobbin which is joined to the bobbin holding the main resistance wire While in other cases, the

trimming resistance is placed on various forms or members external to the main bobbin.

The present invention is directed to wire wound resistors which are to be finally adjusted to a desired value of resistance by the use of trimming resistance wires or material. In accordance with the invention, a number of embodiments of wire wound resistors are provided in which the trimming resistance is located on an integral portion of the bobbin holding the main resistance Wire. In the preferred embodiment of the invention this integral part of the bobbin is an end flange or a flange which separates a plurality of winding channels on a single bobbin. In the preferred embodiment of the invention disclosed, a single bobbin is used to hold a plurality of separate resistors which can be used either individually or connected in parallel, each separate resistor being complete with its own trimming resistance located on a flange on the bobbin.

It is therefore an object of this invention to provide precision wire wound resistors which are finally adjusted to a desired overall resistance value by the use of trimming resistance members.

Another object of this invention is to provide precision wire wound resistors in which the trimming resistance is provided on at least one flange member which is located adjacent to the portion of the bobbin holding the main resistance wire.

A further object of the invent-ion is to provide a novel arangement of a plurality of precision wire wound resistors, complete with individual trimming members on a single bobbin.

Yet another object of the invention is to provide a plurality of precision wire wound resistors and individual trimming resistors on a single bobbin, the whole arrangement being relatively compact and easy to manufacture.

A further object of the invention is to provide a number of wire wound resistors in which trimming resistance members are connected thereto by the use of butt-welding.

Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings in which:

FIGURE 1 is a plan View of an embodiment having a number of individual resistors connected in parallel;

FIGURE 1A is a diagrammatic sketch of the resistance elements of the resistor of FIGURE 1;

FIGURES 1B and 1C are a sectional view taken along lines 113-113 of FIGURE 2 and a top plan view of one of the trimming resistance rings of FIGURE 1 respectively; and

FIGURE 2 is a plan view of another embodiment of resistor having a number of individual resistors connected in parallel;

FIGURE 2A is a diagrammatic sketch of the resistor elements of the resistor of FIGURE 2;

FIGURE 23 is an end view of the resistor of FIGURE 2; and

FIGURE 2C is a top plan view of one of the bobbin flanges.

FIGURES 1, 1A, 1B, 1C and FIGURES 2, 2A, 2B and 2C show two embodiments of resistors in which a number of individual resistors are wound on a single bobbin complete with respective individual trimming resistance members. Referring first to FIGURES 1, 1A and 1C, the bobbin has a number of individual winding channels 16a, 16b, 16c and 16d in which the respective separate main resistance wires 18a, 18b, 18c and 18d are wound around the core of the bobbin. It should be understood that there may be as many winding channels as desired and that the respective main resistance wire 18 in each channel is wound to achieve slightly less than the desired value of resistance for the individual resistor formed in each channel. Each channel 16 has two end flanges, designated 1311 through 13a, so that channel 16a is bounded by flanges 13a and 13b; channel 16b bounded by flanges 13b and 130; etc. A trimming resistance member 60 in the form of a two piece ring is mounted on each of the flanges 13. The trimming members are designated 60a through 6012 to correspond to the respective flanges on which they are mounted. Each of the trimming rings 60, as shown in FIGURE 1A, has two separate half sections designated and r respectively. The upper portion of each half section of each ring is formed with an upwardly turned end 63 as shown most clearly in FIGURE l B. Each end 63 is also provided with a depression 64 to accommodate a jumper bar. The lower portions of the r and 1 sections of rings 60 are separated by an opening 61.

As seen in FIGURE 1A, main resistance wire 18a has one end connected to the end at the opening 61 of the f section of trimming resistance ring 60a. The other end of main resistance wire 18a is connected to the end at the opening 61 of the r section of trimming resistance ring 6017. The preferred butt-welded connections are shown in FIGURE 1C. The other main resistance wires are similarly connected, for example, with wire 18b going from one end of the f section of ring 60b to the r section end of the ring 600. The r section end of ring 60a and the 7 section end of ring 60e at the opening 61, which are the end rings on the bobbin, are left unconnected. Trimming of the resistor [formed by each main resistance wire and its respectively connected f and 1' half sections of the two adjacent trimming resistance rings is accomplished by grind these two half sections of the rings.

When a plurality of individual resistors are to be provided which are to be individually connected to a circuit or a number of circuits, individual leads are connected at the ends 63 of the respective half-ring sections adjacent the opening 62. Thus, to connect the resistor formed by the wire 18a into the circuit, a connection is made to the end 63 of the 1 section of ring 60a and to the end 63 ot the r section of ring 60b. Connections are made to the resistors formed by the other windings 18b, 18c and 18d, and their respective half-section trimming rings in the same manner. These leads are connected by soldering or butt-welding to the appropriate sections of the half-rings. In this manner, a number of separate resistors, complete with their own trimming members, may be Wound on a single bobbin. Where a number of the individual resistors on the bobbin are to be connected in series, the rings 60* on the flanges adjacent the resistor sections to be so connected are not cut at the point 63. Here, taps are brought out at the points 63 of the flanges on the ends of the complete series resistor for connection to be made to the external circuit.

When the resistors on the bobbin of FIGURE 1 are to be connected in parallel, two low resistance, electrically conducting jumper bars 68 and 69 are provided. These bars are soldered or welded to the half-ring trimming sections as shown in FIGURE 1A. The jumper bar 68 is placed in the depression 64 of the r section of each of the rings 60 and soldered or welded. Similarly, the jumper bar 69 is placed in the depression 64 of each of the 1' sections of the trimming rings. As can be seen, all of the individual resistors are now connected in parallel. The jumper bar 68 is connected to the lead 22a while jumper bar 69 is connected to lead 22b. The whole parallel resistor assembly may then be connected into the desired circuit by the leads 22a and 22b.

It should be understood that the resistor of FIGURE 1 has a number of advantages. First of all, a plurality of individual resistors may be wound on a single bobbin in a compact manner. Each resistor has its own individual trimming members to adjust its final value. Where the individual resistors are connected in parallel as shown in FIGURES 1 and 1A, the whole resistor may be slipped into a housing which can then be filled with a suitable insulating fluid such as transformer oil to aid in dissipating the heat. All that would extend through the housing are the two leads 22a and 2212. It should be clear that this arrangement of parallel connected resistors is very compact and easy to assemble.

Referring to FIGURES 2, 2A, 2B and 2C, a single bobbin 11 is shown having a number of individual resistors thereon complete with trimming resistance members. These individual resistors may be connected in parallel in a simple manner. The bobbin 11 has three winding channels 16a, 16b and 160. It should be understood, of course, that any odd number of winding channels 16 may be used. The winding channel 16a is bounded by the flanges 71a and 7112 which correspond to end flanges for the individual resistance winding 18a in channel 16a. The winding channel 16b is similarly bounded by the flanges 71b and 71c and the winding channel 160 bounded by the flanges 71c and 71d. A respective main resistance wire 18a, 18b and 180 is wound on the bobbin core in each of the respective winding channels.

As shown most clearly in FIGURE 2B, each of the flanges 71 is formed with two cut-out portions or grooves 73. Each of the cut-outs 73 is preferably of substantially circular configuration although any other suitable shape may be used. While the cut-outs 73 are shown spaced about 90 apart in the drawing, this is done for illustrative purposes only. In a preferred embodiment of the invention the grooves 73 are made as close together. as possible in order to obtain the maximum amount of trimming resistance. As can be seen in FIGURE 2, the grooves 73 of the flanges 71 are located along two lines which are substantially parallel with the axis of the bobbin core. If desired, the cut-outs 73 may be located along some other type of line, such as curved, slanted and suitably shaped jumper bars provided. This is explained below.

Mounted on each of the flanges 71 is a respective resistance trimming ring 70. These rings are similar in construction and are designated by the numerals 70a, 70b, 70c, 70a to correspond to the respective flanges on which they are mounted. Each of the rings 70 is formed with two depressed portions or loops 74 and 75. The loop 75 preferably has a larger radius than the loop 74. Each ring 70 also has an open portion and two ends designated 77 and 78 respectively. The rings 70 are mounted on the flanges 71 with the loops in the cut-out 73 so that the depressed loops 74 and 75 of different radii alternate from flange to flange. This is shown in FIGURE 2A. The rings 70 can be all constructed the same. Mounting of the rings is effected by reversing the rings for every other flange 71 so that the staggered arrangement of alternate smaller and larger radius loops 74 and 75 is obtained.

The main resistance wire 18a in channel 16a is connected from the end 77 of ring 70a to the end 78 of ring 70!). Similarly, the main resistance wire 18b is connected from the end 77 of ring 70b to the end 78 of ring 70c. Also, resistance wire 18c is connected from the end 77 of ring 700 to the end 78 of the ring 70d. The end 78 of ring 70a and the end 77 of ring 700. are left unconnected. Where more than three winding channels are provided a sufficient number of flanges 70 and rings 71 are provided to connect the main resistance winding in each channel in the same manner.

In order to connect the resistors formed by the respective main resistance wires and their connected rings in parallel, two jumper bars 79 and 80 of low resistance, electrically conductive material are provided. The jumper bar 79 is connected to the ring 70a by placing it in the smaller loop depression 74 and soldering or welding it thereto. The jumper bar 79 passes through the cut-out portion 73 of flange 7111, but does not contact the ring 70b since the larger loop depression 75 is located in this cut-out portion. The jumper bar 79 is electrically connected to the ring 70c since the smaller loop depression 74 is in line with the jumper bar 79 as it is extended across the resistor. In a similar manner the jumper bar 80 is electrically connected to rings 70b and 70d but is not connected to ring 70c. The end of jumper bar 79 is electrically connected to a lead 22a while the end of jumper bar 80 is connected to a lead 22b. Thus, each jumper bar is electrically connected to every other ring 70 so that each resistance winding 18 and its two connected trimming rings 70 are connected to the leads 22a and 22b. Where the cut-outs 73 are not located in a straight line parallel to the bobbin core, which might be desirable in some cases, the jumper bars are constructed accordingly of the desired shape.

As can 'be seen in FIGURE 2A, the resistors formed by windings 18a, 18b, and 180 and their respective trimming sections are connected in parallel. The trimming resistance for the main Winding 18a goes from the connected end 77 of ring 70a around the ring to the point where the jumper bar 79 makes contact therewith. The resistance winding 18a also has a trimming section going from the end '78 of ring 70b to the point 74 where the jumper bar is connected to this ring. In a similar manner, the other resistance windings 18b and 180 have trimming resistances on each end thereof. In order to adjust the final resistance of any one of the resistors formed by the respective main resistance wires 18a, 18b or 180, it is only necessary to grind the respectively connected trimming rings.

It should be recognized that the arrangement for conmeeting the resistors in parallel shown in FIGURE 2 does not entail the cutting of the trimming rings as in embodiment shown in FIGURE 1.

It should also be understood that the rings 7 0 of FIG- URE 2 do not have to be made of trimming resistance material but can be made of copper wire. While this latter arrangement does not provide resistance trimming the resistor of FIGURE 8 using copper wire rings 70 still provides a novel way of connecting a number of resistors on a single bobbin in parallel so that the resistor has a minimum outside diameter over which a protective sleeve can be slipped. The completed assembly can then he slipped into a sleeve, filled with a suitable compound, and epoxy resin applied on each end to form a seal. Of course, the rings 60 of the resistor of FIGURE 1 may also be made of copper rather than resistance material to permit the connection of a number of resistors in parallel in a simple manner.

It therefore can be seen that a number of embodiments of the invention have been disclosed in which wire wound resistors are made to precision specifications in a novel manner. In each of the embodiments a single or a number of resistors are wound on a single bobbin complete with trimming resistance members. These members may be operated upon in a simple manner in order to adjust the resistor to its final desired value. In the embodiments of the invention shown in FIGURES 1 and 2, a novel arrangement is provided for connecting a plurality of resistors on a single core in parallel.

While preferred embodiments of the invention have been described above, it will be understood that these are illustrative only, and the invention is limited solely by the appended claims.

What is claimed is:

1. A wire Wound resistor comprising a bobbin of insulating material having a main body portion and a number of flanges thereon, two adjacent flanges forming a Winding channel therebetween for a respective resistor element on said bobbin, a main resistance wire member wound in each winding channel, an electrically conductive trimming resistance ring on each said flange, each ring being formed by separate first and second sections, and means for electrically connecting the ends of the main resistance wire in each channel to one end of the first section of a trimming ring on the flange on one side of the channel and to one end of the second section of the trimming ring on the flange on the other side of the channel.

2. A wire wound resistor comprising a bobbin of insulating material having a main body portion and a number of flanges thereon, two adjacent flanges forming a winding channel therebetween for a respective resistor element on said bobbin, a main resistance wire member wound in each winding channel, a trimming resistance ring on each said flange, each trimming resistance ring being formed by separate first and second sections, means for electrically connecting the ends of the main resistance wire in each channel to one end of the first section of a trimming ring on the flange on one side of the channel and to one end of the second section of the trimming ring on the flange on the other side of the channel respectively, first means for electrically connecting the other end of all of said first sections of said trimming rings, and second means for electrically connecting the other ends of all of the second sections of said trimming rings, there-by connecting all of said resistor elements in parallel.

3. A wire wound resistor comprising a bobbin of in sulating material having a main body portion and a number of flanges thereon, two adjacent flanges forming a winding channel therebetween for a respective resistor element on said bobbin, a main resistance wire member wound in each winding channel, a trimming resistance ring on each said flange, each trimming resistance ring being formed by separate first and second sections, means for electrically connecting the ends of the main resistance wire in each channel to one end of the first section of a trimming ring on the flange on one side of the channel and to one end of the second section of the trimming ring on the flange on the other side of the channel respectively, a first electrically conductive jumper bar electrically connecting the other ends of all of said first sections of said trimming rings, a second electrically conductive jumper =bar electrically connecting the other ends of all of the second sections of said trimming rings, a first lead connected to first jumper bar and a second lead connected to said second jumper bar, said first and second leads being used to connect said resistor into a circuit.

4. A wire wound resistor comprising a bobbin of insulating material having a main body portion and a plurality of flanges spaced thereon, two adjacent flanges forming a respective winding channel therebetween for a respective resistor element on said bobbin, a main resistance wire member wound in each said channel, a ring of electrically conductive material on each said flange, each ring being formed with an open portion having first and second ends and a first and connecting point, means for connecting an end of each said main resistance wire to the first end of a ring on a flange on one side of the channel and the other end of said main resistance wire to the second end of the ring on the flange on the other side of the channel, and means for electrically connecting said first connecting points of alternate rings on said bobbin.

5. A wire wound resistor comprising a bobbin of insulating material having a main body portion and a plurality of flanges spaced thereon, two adjacent flanges forming a respective winding channel therebetween for a respective resistor element on said bobbin, each of said flanges being formed with two cut-out portions thereon, a main resistance wire member wound in each said channel, a trimming resistance ring on each said flange, e'ach trimming resistance ring being formed with an open portion having first and second ends and first and second depressed portions, the rings being mounted on said flanges with the depressed portions in said flange cut-out portions, means for electrically connecting an end of each of said main resistance wire to the first end of a trimming ring on a flange on one side of the channel and the other end of said main resistance wire to the second end of the trimming ring on the flange on the other side of the channel, and means for electrically connecting the first depressed portions of alternate rings on said bobbin.

7 o 6. A wire wound resistor comprising a bobbin of insulating material having a main body portion and a plurality of flanges spaced thereon, two adjacent flanges forming a respective winding channel therebetween for a respective resistor element on said bobbin, each of said flanges being formed with first and second cut-out portions, a main resistance wire member Wound in each said winding channel, a trimming resistance ring on each said flange, each trimming resistance ring being formed with an open portion having first and second ends and first and second depressed portions having respectively different sizes, the trimming resistance rings on said flanges located with the depressed portions in said flange cut-out portions, means for connecting an end of each said main resistance wire to the first end of a trimming ring on a flange adjacent one side of the winding channel and the other end of said main resistance Wire to the second end of the trimming ring on the flange on the other side of the channel, a first jumper bar electrically connecting said depressed p01- tions of said first size of a selected first group of rings and passing through said depressed portion of second size of selected rings which are not connected by said first jumper bar, a second jumper bar electrically connecting said depressed portions of said first size of the rings not connected by said first jumper bar and passing through said depressed portions of second size of selected ones of said selected first group of rings, a first lead electrically connected to said first jumper bar and a second lead electrically connected to said second jumper bar, said leads being used to connect said resistor into a circuit.

7. A wire wound resistor comprising:

a bobbin of insulating material,

a plurality of main resistance Wire members on said bobbin,

a plurality of trimming resistance members also on said bobbin,

means electrically connecting each of said main resistance members to at least one of said trimming resistance members,

and means for electrically connecting each of said main resistance members and its respectively connected trimming resistance member in parallel.

8. A wire wound resistor as set forth in claim 7 wherein the diameters of the trimming resistance members are greater than the diameters of the main resistance wire members.

9. A wire wound resist-or as set forth in claim 8 wherein the means for connecting the main resistance wire members to the trimming resistance members includes a butt- Welded connection.

10. A wire wound resistor comprising:

a bobbin of insulating material,

a plurality of main resistance wire members on said bobbin,

a plurality of trimming resistance members also on said bobbin,

means for electrically connecting each end of a main resistance wire member to a trimming resistance member to form a separate resistor assembly complete with trimming resistance members on each end thereof,

and means for electrically connecting selected ones of said separate resistor assemblies together.

11. A Wire Wound resistor as set forth in claim 10 wherein said last named means electrically connects a plurality of said separate resist-or assemblies together in parallel.

12. A wire wound resistor as set forth in claim 10 wherein said last named means includes two wires electrically connected to the trimming resistance members of the separate resistor assemblies.

13. A wire wound resistor as set forth in claim 10 wherein the diameters of the trimming resistance members are greater than the diameters of the main resistance wire members and the means for electrically connecting respective ones of said members together includes a buttwelded connection.

14. A wire wound resistor comprising:

a bobbin of insulating material having a body formed with a plurality of winding channels and auxiliary portions thereon,

a respective main resistance wire member wound in selected ones of said winding channels,

a trimming resistance member located on selected ones of said body auxiliary portions,

means connecting selected ones of said main resistance Wire members between two of said trimming resistance members to form a plurality of separate resistance assemblies,

and means for connecting a plurality of said separate resistor assemblies in parallel.

15. A wire wound resistor as set forth in claim 14 wherein said last named means includes two wires which are electrically connected to selected ones of the trimming resistance members of said separate resistor assemblies.

16. A wire wound resistor comprising:

a bobbin of insulating material,

a plurality of separate main resistance wire members wound on said bobbin,

a trimming resistance member located on said bobbin between each adjacent pair of main resistance Wire members and on the outside of the end main resistance wire members,

means for electrically connecting the end of each main resistance wire member to one of the adjacent trimrning resistance members,

and means for electrically connecting selected ones of said trimming resistance members together.

17. A wire wound resistor comprising:

a bobbin of insulating material,

a plurality of separate main resistance wire members wound on said bobbin,

a discontinuous electrically conductive connecting member located on said bobbin between each adjacent pair of main resistance wire members and on the outside of the end main resistance wire members,

means for electrically connecting the end of each main resistance wire member to one end of the connecting member adjacent the said end,

and means for electrically connecting said discontinuous connecting members together to place said main resistance wire members in parallel.

18. A wire wound resistor comprising:

a bobbin of insulating material having at least two areas for mounting a main resistance element thereon,

a main resistance element wound on each said area,

an electrically conductive member on said bobbin on each side of each said area mounting a resistance element, one said member serving as a common member between adjacent areas, each said member being formed by separate first and second sections,

means electrically connecting one end of the main resistance element of each area to said first section of a conductive member located at one side of the respective area and the other end of said element to said second section of said conductive member located at the other side of a respective area,

and means for electrically connecting said all of said first sections of said conductive members together and all of said second sections together to connect the resistance elements in parallel.

19. A wire wound resistor comprising:

a bobbin of insulating material having at least two areas for mounting a main resistance element thereon,

a main resistance element wound on each said area,

an electrically conductive trimming resistance member on said bobbin on each side of each said area mounting a resistance element, one said member serving as a common member between adjacent areas, each said member being formed by separate first and second sections,

means electrically connecting one end of the main resistance element of each area to a point adjacent one end of a said first section of a trimming resistance member located at one side of the respective area and the other end of said element to a point adjacent one end of said second section of said conductive member located at the other side of a respective area,

means electrically connected to a point adjacent the other ends of all of said first sections of said trimming resistance members,

and means electrically connected to a point adjacent the other ends of all of said second sections of said trimming resistance members whereby said main resistance elements are connected in parallel.

20. A wire Wound resistor comprising:

a bobbin of insulating material having at least tWo areas for mounting a main resistance element thereon,

a main resistance Wire element Wound on each said area,

an electrically conductive member on said bobbin on each side of each said area mounting a resistance element, one said member serving as a common member between adjacent areas, each of said members being discontinuous and having first and second ends,

means for electrically connecting one end of each said main resistance element to a point adjacent the first in end of a conductive member on one side of a respective area and the other end of each said main re sistance element to a point adjacent the second end of said member on the other side of respective area, first means for electrically connecting alternate conductive members, and second means for electrically connecting the other alternate conductive members unconnected to said first means to place said resistance elements in parallel.

References Cited by the Examiner UNITED STATES PATENTS RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner. 

1. A WIRE WOUND RESISTOR COMPRISING A BOBBIN OF INSULATING MATERIAL HAVING A MAIN BODY PORTION AND A NUMBER OF FLANGES THEREON, TWO ADJACENT FLANGES FORMING A WINDING CHANNEL THEREBETWEEN FOR A RESPECTIVE RESISTOR ELEMENT ON SAID BOBBNIN, A MAIN RESISTANCE WIRE MEMBER WOUND IN EACH WINDING CHANNEL, AN ELECTRICALLY CONDUCTIVE TRIMMING RESISTANCE RING ON EACH SAID FLANGE, EACH RING BEING FORMED BY SEPARATE FIRST AND SECOND SECTIONS AND MDANS FOR ELECTRICALLY CONNECTING THE ENDS OF THE MAIN RESISTANCE WIRE IN EACH CHANNEL TO ONE END OF THE FIRST SECTION OF A TRIMMING RING ON THE FLANGE ON ONE SIDE OF THE CHANNEL AND TO ONE END OF THE SECOND SECTION OF THE TRIMMING RING ON THE FLANGE ON THE OTHER SIDE OF THE CHANNEL. 